Vacuum insulation

    The purpose of vacuum insulation is to eliminate gas convection heat exchange, reduce gas heat conduction and heat exchange, and reduce radiation heat exchange. There are three main forms of vacuum insulation: high vacuum insulation, vacuum porous insulation, and high-quality multilayer insulation.

    It is generally believed that as long as the vacuum degree of the adiabatic space is better than 1.3×10^-2Pa, the gas convection heat transfer can be eliminated. In this state, radiation heat exchange is mainly used, followed by gas molecule heat conduction.

    In high vacuum, radiation heat exchange is mainly used. In order to reduce the loss of radiation heat exchange, the following measures can be taken:

    1. Choose low-emissivity materials to make low-temperature containers, such as copper, aluminum, etc., or coat the surface of the material with low-emissivity materials, such as silver, copper, aluminum, gold, etc.;

    2. Material surface cleaning and smoothing treatment to reduce the emissivity of the material surface;

    3. Set up multi-layer radiation protection screen to reduce radiant heat.

    Vacuum porous insulation is a kind of insulation method in which porous insulation materials, such as powder or fiber materials, are filled in the insulation space, and then the space is pumped to a certain degree of vacuum. The vacuum degree of adiabatic space is usually 1~10Pa. This kind of insulation method is widely used in low temperature technology.

    There are many factors that affect vacuum porous insulation, mainly in the following aspects.

    1. The type of filling material and the type and pressure of the gas in the insulation layer.

    2. The effect of filling material density and particle diameter on effective thermal conductivity. When the density of the filling material is generally 150~200kg/m3, the heat insulation effect is the best. The most suitable particle diameter is 10-50μm.

    3. The effect of filler metal powder on effective thermal conductivity. Filling with metal powder can improve thermal insulation performance. A proper amount of filler metal powder is required. Excessive filling will increase the thermal conductivity of the metal powder, and too little filling will increase the radiant heat flow, generally 30% to 50%.

    High-vacuum multilayer insulation is currently the best insulation method, and it is widely used in low-temperature technology. The materials used for the heat-insulating reflector include aluminum foil, copper foil, and metalized polyester film. But at present, the most commonly used are alkali-free glass fiber cloth, glass fiber paper, and nylon mesh cloth. Or these materials are used alternately with aluminum foil.

(Huiputech)